The present invention relates to a treatment for improving the quality of a rolled steel rod or bar and especially a concrete-reinforcing rod or bar.
The process can be applied to both killed and semi-killed steels and rimming steels.
The main qualities looked for in a steel rod or bar are, inter alia, as high as possible a `yield point` for the grade of steel used, as well as weldability, fatigue resistance, and sufficient ductility for the intended use.
To improve the weldability and ductility of steel, it is necessary to reduce its carbon and manganese contents, which, however, causes a reduction in its tensile strength. In order to remedy this disadvantage, the steel can be subjected to a suitable cooling treatment, preferably applied directly at the exit of the rolling mill, which enables the `yield point` of the rod or bar to be raised to some extent.
When a rod or bar (such as concrete-reinforcing rod or bar) is cooled by convection or radiation, the law of cooling almost only depends on its diameter, which has the result that, for a bar of a specific diameter, it is necessary in order to modify its elastic limit, to use other means in order to complement the pure and simple act of cooling.
Of these means, it is particularly worth mentioning the addition of dispersoidal elements (e.g., Nb and V) which cause grain refinement and precipitation hardening of the ferrite. This is indeed efficacious but has the disadvantage that the cost increases as the `yield point` is raised.
Thus what is desired is a process which avoids the aforesaid disadvantage without the carbon and manganese contents in the steel having to be increased in a way which is unacceptable from the point of view of weldability.